1. Field of the invention
The invention concerns a non-destructive eddy current test device using additive flux/subtractive flux switching.
2. Description of the prior art
A device of this kind is used, for example, to detect defects or foreign bodies or variations of composition in a metal part. It uses a method based on generating eddy currents at a point in the part under test, subjecting this point to an alternating magnetic field and detecting variations of the eddy currents relative to a reference by detecting variations in the impedance of at least one coil used to generate the eddy currents.
The method is usually a differential method entailing generating eddy currents at two adjacent points in the same part using two identical coils carrying the same current either in phase or 180.degree. out of phase and measuring the difference between the impedances of the two coils. There is no difference if the two points on the part under test have the same composition, even if the temperature is different, and no previous calibration is required. There is a difference if the composition of the part is not exactly the same at the two points at which the coils are located.
As the two coils are located at adjacent points, their magnetic fluxes add or subtract according to the direction of the current in each of the two coils and the direction in which each of the two coils is wound. The two coils constitute a probe which may be remote from the remainder of the test device. Each coil is connected to the remainder of the test device by a coaxial cable which can be up to 200 meters longs. Experience shows that it is sometimes possible to obtain greater resolution by unplugging and interchanging the two terminals of one coil to reverse its flux.
This method of additive flux/subtractive flux switching is not very practical and may even be virtually impossible to use if the coils are very far from the remainder of the test device and are virtually inaccessible. An aim of the invention is to remedy this drawback of prior art devices.